Historically, polyethylene terephthalate has been difficult to mold at molding temperatures below about 110.degree. C. because its crystallization rate is so slow and uneven that warped articles are obtained. Moreover the articles tend to stick to the mold and are difficult to remove. Recently, it has been found that polyethylene terephthalate articles of good quality can be obtained by molding at lower temperatures, e.g., 80.degree.-100.degree. C., if certain materials are added to the resin prior to molding. These additive materials increase the rate of crystallization, and molded articles having smooth glossy surfaces that are easily removable from molds, are obtained when the materials are employed. The additive materials are (1) a sodium or potassium salt of a selected hydrocarbon acid or a sodium or potassium salt of a selected organic polymer containing pendant carboxyl groups and (2) a selected low molecular weight organic compound that is an ester, ketone, sulfone, sulfoxide, nitrile or amide.
However, when flame-retardant systems were employed with these improved crystallization rate materials in polyethylene terephthalate blends, it was found that articles molded from the blends either tended to exhibit inadequate thermal stability or the flame-retardant system did not operate effectively.
The thermal stability requirements for a flame-retardant composition of commercial value are such that PET molding resins need to be stable at a melt temperature of at least 292.degree. C. for a minimum of eight minutes or more. Many applications are even more demanding. A common flame-retardant system comprises a halogenated organic compound and antimony oxide which acts as a synergist. It was determined that the antimony oxide synergist was causing the blend to become thermally unstable when molded at moderate hold-up times, as evidenced by a decrease in physical property values and by high melt flow. Poor thermal stability was also found with antimony pentoxide and a variety of coated antimony oxides which were tested. On the other hand, a number of materials sometimes used as synergists including zinc oxide, zinc sulfide, zinc borate, stannous oxide, molybdenum oxide and molybdates were found not to be effective in polyethylene terephthalate.
A synergist that does not affect heat stability while operating efficiently is desirable, and is provided by this invention.